BBC 6Music has issued a warning to daytime presenter George Lamb, after he appeared to pledge his support on air for Boris Johnson in the London mayoral race.

Lamb, who last night won the inaugural "rising star" prize at the 2008 Sony Radio Academy awards, has been warned by management for breaching impartiality guidelines, a spokeswoman for the digital station said.

On his mid-morning show on May 1 - the day of the local elections - Lamb told listeners: "I know who I'm going to vote for, who has blond hair."

"The problem is it breached BBC editorial guidelines on impartiality," the 6Music spokeswoman said.

"It wasn't a breach of [the Ofcom broadcasting code] as there was with James Whale. This was a breach of the guidelines which we do take seriously."

The corporation's editorial guidelines state: "Our commitment to impartiality and fairness is under intense scrutiny when we report election campaigns.

"Content producers must take responsibility for achieving due accuracy and impartiality in their own output."

News of Lamb's warning comes less than a week after veteran talkshow host James Whale had his contract with TalkSport terminated after he urged listeners to vote for Johnson, the victorious Tory candidate.

TalkSport owner UTV said James Whale's comments, made on March 20, amounted to "a gross error of judgment" and were "totally unacceptable".

But a BBC 6Music spokeswoman said the Lamb incident was less serious than Whale's mistake.

She said the BBC complaints unit had received six complaints about the former T4 presenter's comments.

Ofcom has received three complaints about Whale's comments on TalkSport, and is currently investigating the incident.

Lamb joined the BBC digital-only station last September, and was moved to weekday mid-mornings in October, replacing Gideon Coe, who moved to late nights.

He has sharply divided opinion among 6Music listeners, with supporters and critics setting up pro- and anti-Lamb websites.

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· This article was amended on Monday May 26 2008. Six listeners rang the BBC to complain about 6Music presenter George Lamb's apparent on-air support of Boris Johnson. We originally said that nobody had complained. This has been corrected.